Self-reacting antibodies (autoantibodies) are widely present in a normal immune system. At the same time, a variety of autoantibodies have been shown to be responsible for disease induction. However, molecular characterization of pathogenic as opposed to natural autoantibodies has resisted previous efforts by different investigators. We believe that in order to understand this critical distinction, the autoantibodies to be studied should be tested for their pathogenic potential in such a way that an actual structure-function relationship could be established. Furthermore, current limitations that severely hamper the study of human antibody repertoires must be circumvented. We propose to meet both requirements by applying newly developed antibody cloning technology to the study of myasthenia gravis (MG). Autoantibodies in MG are directed against the acetylcholine receptor, they are clearly responsible for disease induction and in contrast to other autoantibodies, their pathogenic potential can be tested both in vitro and in vivo. The ability of E. coli to correctly assemble and express functional Fab antibody fragments retaining antigen-binding potential has been applied to the development of powerful expression vectors that allow us to explore very large repertoires with much greater efficiency and lower cost than conventional hybridoma technology. We propose to apply this powerful system to the study of antibody repertoires in MG in order to elucidate the molecular basis of pathogenic autoantibodies. We also hope that this kind of approach will provide valuable information regarding the molecular basis of B cell tolerance.